1) Field of the Invention
The present invention relates to a light source unit and a projector, and more specifically, relates to a light source unit that uses a light emitting diode (hereinafter, “LED”).
2) Description of the Related Art
Conventionally, several technologies have been proposed for a light source unit used for a liquid-crystal-type spatial-light-modulator in a projector. Japanese Patent Application Laid-Open Nos. 2003-98483 and 2000-221499 disclose technologies that the reflecting-type polarizing plate and the phase plate are used together for polarization conversion.
Japanese Patent Application Laid-Open No. 2003-98483 discloses a lighting system. In the lighting system, the reflecting-type polarizing plate and the phase plate are provided on an emission plane of a rod integrator. The light is reflected by the reflecting-type polarizing plate, then is reflected by a reflecting surface, which is arranged inside the rod integrator and on the incident side of the rod integrator, and travels toward the reflecting-type polarizing plate again. It is preferable in this lighting system to increase the area of the reflecting surface provided in the rod integrator as much as possible so that the light travels efficiently toward the reflecting-type polarizing plate. However, the amount of light that enters into the rod integrator from a light source decreases when the area of the reflecting surface increases. On the contrary, one approach to increase the amount of the light that enters into the rod integrator from the light source is to make the opening of the rod integrator large. However, in this case, the area of the reflecting surface decreases and hence less light travels toward the reflecting-type polarizing plate.
Japanese Patent Application Laid-Open No. 2000-221499 discloses a light source for image display. This light source includes a reflecting-type polarizing plate and a phase plate right behind a light emitting unit. The phase plate and the reflecting-type polarizing plate are laminated on the light emitting unit. An advantage of this technology is that diffusion of light from the light emitting unit is prevented and loss of light decreases as the reflecting-type polarizing plate and the phase plate are provided in a position closer to the light emitting unit. Therefore, this configuration enables high light usability. However, when the phase plate is formed of an organic compound, the phase plate is sometimes damaged due to the heat from the light emitting unit. If the phase plate is damaged, it becomes difficult to perform normal polarization conversion. Particularly, when electric current is injected up to the rated limit to obtain the amount of light as much as possible from a small light emitting unit, the temperature of the light emitting unit may rise to a high temperature. Therefore, polarization conversion may fail even if the reflecting-type polarizing plate and the phase plate are provided adjacent to the light emitting unit.
Further, Japanese Patent Application Laid-Open No. 2002-319708 discloses a technology for supplying bright light. Some LEDs have a substrate in the light emitting unit, and the light is supplied via the substrate. For example, an LED that has the flip-chip-mounted light-emitting unit generates light in a semiconductor layer below a sapphire substrate. The light comes from the semiconductor layer, passes through the sapphire substrate directly or passes through the sapphire substrate after being reflected by an electrode layer, and is outputted. When the light enters into an interface between the air and the sapphire substrate at an angle equal to or larger than a critical angle, the light is totally reflected on the interface and taken into the light emitting unit. Thus, the light taken into the light emitting unit is partially absorbed while repeating the total reflection and reflection on the electrode layer. This technology is proposed to overcome the above problem. In this technology, the light taken into the light emitting unit is decreased for supplying the bright light.
Japanese Patent Application Laid-Open No. 2002-319708 discloses a technology for improving the efficiency of the light source unit. In this technology, protrusions and recessions of about 1 micrometer are formed on a surface on the emission side of the sapphire substrate. The total reflection on the interface between the sapphire layer and the air is reduced by profiling the sapphire substrate, and thereby the efficiency of the light source unit improves. However, a disadvantage of this technology is that profiling is difficult since the sapphire substrate is very hard, and it is not easy to perform chemical processing such as oxidation. Therefore, another technology is still required to easily decrease the light taken into the light emitting unit without performing processing of the sapphire substrate.
In another technology, the total light reflection on the interface of the sapphire substrate is reduced by bonding another transparent member on the sapphire substrate. In general, the sapphire substrate and another transparent member are bonded by using an adhesive. However, when the sapphire substrate and another transparent member are bonded together, and the sapphire substrate and another transparent member thermally expand independently due to the heat from the light emitting unit, a distortion occurs in the bonded portion. The light emitting unit may be damaged due to the distortion. Particularly, the output of the LED is getting larger these days, and it is thereby conceivable that deterioration in such a bonded portion becomes notable. Therefore, still another technology is required for reducing the total light reflection on the interface of the sapphire substrate.
As explained above, it is difficult in the conventional technologies to supply light efficiently.